Work is produced from heat in a continuous cycle. A flow of first working fluid is provided to a high pressure boiler to produce a flow of first working fluid vapor. A second working fluid in vaporous form is compressed, after which a third working fluid is formed by mixing the first working fluid vapor and the second working fluid. Thermal energy is transferred directly between the first and second working fluids in the mixing chamber exclusive of any intervening structure. A refrigeration loop containing a fourth working fluid extracts thermal energy from a low grade thermal energy source and moves the thermal energy to the first working fluid and/or the second working fluid.